1. Field of the Invention
This invention relates to solid state image sensors, and more particularly, to active pixel sensor (APS) technology.
2. Description of Related Art
Active pixel sensors and charge coupled devices (CCD) are solid state photosensitive devices which are commonly constructed as an array of photosensitive cells, each cell in the array corresponding to a pixel. A typical application for CCD or APS image sensing arrays is in a digital camera or other type of image sensor.
One advantage to APS devices over CCD devices is that APS technology is more compatible with metal oxide semiconductor (MOS) technology. This allows the support electronics needed to read signals from the APS array, and to process those signals, to be constructed on the same chip and at the same time as the APS array itself. This can significantly reduce the total cost of an APS technology based imaging device.
A basic prior art APS device comprises a reverse-biased photosensitive region of semiconductor material that absorbs incident electromagnetic radiation and produces hole-electron pairs. The electrons generated by the incoming light are collected and held in the photosensitive region by the action of a pin diode formed between a pinning layer at the incident surface of the device and the semiconductor material in the photosensitive region.
Incoming electromagnetic radiation first passes through the pinning layer and then into the photosensitive region. Holes generated when the incoming electromagnetic radiation is absorbed are collected and removed from the photosensitive region by the pinning layer and the photodiode formed between the reverse-biased photosensitive region and the substrate. The pinning layer also serves to isolate the stored electrons from the semiconductor surface, which is known to provide significantly more sites for recombination than the silicon bulk.
Electrons generated when the incoming electromagnetic radiation is absorbed in the photosensitive region remain trapped in the photosensitive region until a transfer device removes them. The transfer device is typically a polysilicon gate and an adjacent semiconductor region. The polysilicon gate can be triggered by the application of a potential source to allow current flow between the photosensitive region and the adjacent semiconductor region. The number of electrons trapped in the photosensitive region relates to the intensity of the absorbed electromagnetic radiation and to the duration of exposure of the APS device to the incoming radiation.
Thus, the current flow which occurs when the transfer device is activated determines the brightness at the pixel corresponding to the APS device. With multiple APS devices in an array, each one corresponding to a single pixel, a multiple pixel image can be built up by scanning the APS array and activating the transfer device for each cell to determine the brightness of the image at each pixel.
One difficulty with APS designs is that the amount of charge that can be collected and held in the pin diode is limited by the total reverse potential of the diode. If the APS cell is strongly overexposed, the diode electron charge collected will exceed this limit and forward bias the pin diode. Excess electrons will then spill out and disturb adjacent APS cells. When excess electrons from one cell spill over into adjacent cells, causing those cells to also appear to be strongly illuminated, the disturbance is referred to as "blooming". The excess electrons from overexposed APS pixels can also interfere with the proper operation of other devices on the substrate.
To solve the blooming problem, anti-blooming gates have sometimes been used, however this increases cost and device complexity.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a photosensitive device including an APS array having excellent isolation between adjacent APS devices in the array.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.